Improve RF harmonic measurements with a PXI digitizer

For RF test and production engineers who need to make optimized and repeatable high-speed harmonic measurements, this article describes how to improve harmonic measurements using a 13 GHz PXI-based RF digitizer. This is achieved using external filtering to reject the fundamental carrier. An example is presented showing how the power level presented to the RF digitizer may be managed. Introduction

Using a 13-GHz PXI-based RF digitizer extends your measurement capability for characterizing high frequency device performance. Devices such as amplifiers often require measurement of harmonics. By using a high frequency RF digitizer with some simple additional RF components, you can make demanding low-level harmonic measurements while keeping test time to a minimum—a vital consideration for high volume manufacturing test applications. This article describes the measurement challenges involved in measuring low-level harmonics and presents a practical example test set-up.

The measurement challenge

Ongoing improvements in RF component design mean that harmonic and spur levels are getting lower (relative to the carrier) and start to stretch the spurious free dynamic range (SFDR) of measurement instruments. To ensure that you are truly measuring these levels—and not the measurement equipment’s own products—it is important to properly manage the signal levels incident to your measurement equipment. In that regard, it is important to understand how the instrument architecture will affect the measurement. Signal analyzers and digitizers predominantly use the same approach to measurement—they down-convert and then measure at a lower frequency.

Simplified down converter

The instrument’s input RF paths have limitations that depend upon combinations of input power levels and frequencies being measured. These are typically expressed as Third Order Intercept (TOI) points and Second Order Intercept (SOI) points. Usually, the RF input of the equipment features switchable attenuation. The switchable attenuation manages the power level related to the down-converter to reduce the limitations, with the compromise of increasing the noise floor.

Dynamic range

To measure low-level signals, input attenuation should be reduced as much as possible to reduce the measurement noise floor and increase the signal-noise (S/N) ratio. Well-designed PXI-based RF digitizer modules utilize a simple architecture that excludes front-end pre-selection. High-level out-of-band signals can over-drive the digitizer input stages and create spurious signals. Increasing input attenuation can mitigate this, but will reduce S/N ratio for the measurement of the low-level signal of interest.

Clearly, for a harmonic measurement there is a high-level signal (the fundamental) and a low-level signal (the harmonic)—both are present at the input to the RF digitizer. Input attenuation would need to be set for the higher of these power levels, reducing the S/N ratio for the low-level signal.

With reduced S/N ratios, measurements may be achieved by using narrower resolution bandwidth settings, increased averaging, and reduced sampling rates. You will find that this increases measurement times, may be impractical, or the level may simply be too low to be measured. By use of a simple external filter (high pass, band pass, band stop, etc.) To reject the fundamental, the input attenuation may then be optimized for the low-level harmonic. This improves the S/N ratio, allowing higher sampling rates and reduced averaging for improved measurement times.